The present invention relates to the field of digital signal processing apparatus which operates upon a one-dimensional array of input digital values. The digital values may be real, imaginary or complex and are typically amplitudes at a series of frequencies obtained by spectrum analysis measurements, corresponding values of a derived functions such as a transfer function or coherence. A particular and commonly used array consists in the results of applying discrete Fourier transform (DFT) techniques and in such a case the values pertain to equally spaced frequency values, that is to say the frequency axis (in general terminology, the independent axis) is linear. However it is often desired to present the results on a logarithmic frequency axis in order to permit a broad frequency range to be presented on a single display, or to enhance spectral features such as slopes which are multiples of 3 dB/octave, or to assist the identification and estimation of features such as breakpoint frequencies, noise levels, etc.
In order to display the array of values they are applied by way of a digital-to-analog converter (DAC) to a display device synchronously with control of the display device to sweep along the independent axis. The display device may be a cathode ray tube (CRT) or other device providing an ephemeral display or it may be some form of a recorder or plotter which produces a permanent record, whether of graph or histogram form.
In the past, a logarithmic independent axis has been obtained by providing a logarithmic sweep on this axis, e.g. in a CRT or XY plotter. This has at least two disadvantages. In the first place it precludes the use of simpler, less expensive display devices with linear sweep, e.g. a CRT with electromagnetic deflection. In the second place, if the independent axis has a large span, e.g. as with a broad-band spectrum analysis, the values at the low end of the independent axis become very widely spaced.
The main object of the invention is to overcome the first disadvantage, while a development of the invention enables the second disadvantage also to be overcome.